1. Field of the Invention
The invention relates to a method for operating a multi-stage electrical heater comprised of several heating elements.
2. Description of Related Art
A multi-stage electrical heater, e.g., a heater with a positive resistance temperature coefficient PTC, is formed of several individual elements, which often are referred to as heating rods, and which are electrically connected in parallel. Each heating element can in turn consist of several sub-elements, i.e., individual PCT blocks.
Each individual heating element can be activated or deactivated via a switch, for example, an electronic switch. The power PH consumed in a heating element, i.e., the supplied electrical power equal to the emitted thermal power, depends on the electrical resistance RH of the heating element at the working point at a preset operating voltage UB.PH=(UB)2/RH
However, the electrical resistance RH of the heating elements is subjected to high variations owing to the production process. As a result, the respective power emitted by the heating elements scatters as well. In order to satisfy the requirement of, for example, air-conditioning system manufacturers on a specific power at a set working point, extensive measures, e.g., compensating or sorting, are hence necessary to maintain the electrical resistance RH of the individual heating elements required for the working point.
A heater composed of several heating elements may satisfy the requirement for a specific overall power, but the heating power of the individual heating elements may vary. Due to the heating power generally emitted over a larger surface, this results in a formation of temperature layers of air streaming out of the heater. The heated air has noticeable temperature differences over the outlet surface. This is undesired, for example, in heating or air-conditioning systems, since it leads to irregularities in how the temperature in a heated space, e.g., the interior of a vehicle, is controlled. As a consequence, all individual heating elements of a heater should consume or emit the same power.